A frequency band of several dozen to several hundred MHz is applied to conventional mobile communication devices such as automobile radio transmission devices. However, a system operating with a 1.5 GHz or higher sub-microwave band is becoming mainstream, fulfilling the increasing demand for mobile communication devices. Surface acoustic wave (SAW) elements utilizing surface acoustic waves generated at a solid surface are applied mainly as filters to most mobile communication devices. The SAW element generally applies an electric wave to an inter-digital transducer fixed on the surface of a piezoelectric substrate, thus driving a surface acoustic wave to the surface of the substrate. Crystal having a surface acoustic wave with a large acoustic velocity is used as a substrate for the SAW filter found on the market, and an Al inter-digital transducer used for transmitting the surface acoustic wave is formed on the crystal substrate.
As a conventional frequency adjusting method for SAW filters, a silicon oxide (Si--O) thin film having a mismatching lattice configuration is formed on the substrate, and light is irradiated to the substrate so as to change the transmission velocity of the surface acoustic wave. This method is disclosed in Published examined (Tokkohei) Japanese Patent Application No. Hei 3-51128.
Operation frequencies for a surface acoustic wave module including SAW filters depend on the acoustic velocity of the surface acoustic wave of a piezoelectric body, and the configuration cycles of inter-digital transducers used for receiving and transmitting surface acoustic waves. For instance, cellular phones have a 12.5 kHz channel gap for a communication circuit, so that the operation frequencies of SAW filters are required to be highly precise. However, in conventional methods of manufacturing SAW filters, the precision of a film thickness (from several nm to 500 nm) and of a configuration cycle (from submicron to several u m) is only within 1%. It is difficult to tune 1.5 GHz frequency to .+-.12.5 kHz preferable frequency, and the yield of products manufactured through a strict quality control is 50-70%. A method for obtaining the operation frequency of SAW filters in minute variations is desired; the above-mentioned Published examined (Tokkohei) Japanese Patent Application No. Hei 3-51128 is not sufficient to satisfy this request.
Another problem is that the frequencies change greatly, namely, about several hundreds ppm/.degree. C., so the device cannot be practical.
And in the step of manufacturing SAW elements, the operation frequency of the elements changes irregularly before and after the final step, namely, the mounting of sealing window of packaging. This is a serious problem that has not been solved yet.
Moreover, when a substrate having high pyroelectric properties such as LiNbO.sub.3, LiTaO.sub.3 and Li.sub.2 B.sub.4 O.sub.7 is applied for a SAW filter, the operation frequency changes after a 200-300.degree. C. temperature flow test and the transmission loss of the element increases. Another problem is also found in that a wafer used for the substrate is charged during a separation process (dicing process) after forming electrodes on a sheet so that a potential difference as well as sparks are generated between the electrodes, shunting a section between the electrodes.